The demand for iso-alkenes has recently increased. For example, relatively large amounts of isobutene are required for reaction with methanol or ethanol over an acidic catalyst to produce methyl tert-butyl ether (MTBE) or ethyl tert-butyl ether (ETBE) which is useful as an octane enhancer for unleaded gasolines. Isoamylenes are required for reaction with methanol over an acidic catalyst to produce tert-amyl methyl ether (TAME). With passage of the Clean Air Act in the United States mandating increased gasoline oxygenate content, MTBE, ETBE and TAME have taken on new value as clean-air additives, even for lower octane gasolines. Lead phasedown of gasolines in Western Europe has further increased the demand for such oxygenates.
An article by J. D. Chase, et al., Oil and Gas Journal, Apr. 9, 1979, discusses the advantages one can achieve by using such materials to enhance gasoline octane. The blending octane values of MTBE when added to a typical unleaded gasoline base fuel are RON=118, MON=101, R+M/2=109. The blending octane values of TAME when added to a typical unleaded gasoline base fuel are RON=112, MON=99, R+M/2=106. Isobutene (or isobutylene) is in particularly high demand as it is reacted with methanol to produce MTBE.
The addition of shape-selective zeolite additives such as ZSM-5 to cracking catalysts, e.g., those used in fluidized catalytic cracking (FCC), is beneficial in producing gasoline boiling range product of increased octane rating. However, increased amounts of olefins result, including n-butenes, creating a need for their conversion to higher value products such as isobutene which can be used to produce MTBE.
Butene exists in four isomers: butene-1, cis-butene-2, its stereo-isomer trans-butene-2, and isobutene. Conversions between the butenes-2 is known as geometric isomerization, whereas that between butene-1 and the butenes-2 is known as position isomerization, double-bond migration, or hydrogen-shift isomerization. The aforementioned three isomers are not branched and are known collectively as normal or n-butenes. Conversion of the n-butenes to isobutene, which is a branched isomer, is widely known as skeletal isomerization.
The reaction of tertiary olefins with alkanol to produce alkyl tertiary alkyl ether is selective with respect to iso-olefins. Linear olefins are unreactive in the acid catalyzed reaction, even to the extent that it is known that the process can be utilized as a method to separate linear and iso-olefins. The typical feedstream of FCC C.sub.4 or C.sub.4 + crackate used to produce tertiary alkyl ethers in the prior art which contains normal butene and isobutene utilizes only the branched olefin in etherification. This situation presents an exigent challenge to workers in the field to discover a technically and economically practical means to utilize linear olefins, particularly normal butene, in the manufacture of tertiary alkyl ethers.
In recent years, a major development within the petroleum industry has been the discovery of the special catalytic capabilities of a family of zeolite catalysts based upon medium pore size shape selective metallosilicates. Discoveries have been made leading to a series of analogous processes drawn from the catalytic capability of zeolites in the restructuring of olefins.
Despite these efforts, the skeletal isomerization of olefins e.g., to produce isobutene, has been hampered by relatively low conversion and/or selectivity to isobutene perhaps owing to the lability of these olefins. It is further known that skeletal isomerization becomes more difficult as hydrocarbons of lower molecular weight are used, requiring higher temperatures and lower linear olefin partial pressures.
Generally, the conversion of n-butenes to iso-butene is conducted at selectivities below 90%. In order to obtain higher selectivities, operation at high temperatures (&gt;500.degree. C.) and with high feed dilution (butene partial pressure, typically less than 5 psia (34.5 kPa)) is generally required. Selectivities of greater than 85%, 90%, 95% or even 99% are highly advantageous in commercial conversion of n-butenes to isobutene in order to avoid the need to separate out materials other than n-butene from the product stream. Such high selectivities will permit direct (cascading) or indirect introduction of the isomerizer effluent to an etherification zone where isobutene is reacted with alkanol to produce alkyl tert-butyl ether, e.g., MTBE. Unconverted n-butenes in the isomerizer effluent can be withdrawn either before the etherification zone or preferably, from the etherification zone effluent insofar as the etherification reaction utilizes only the isobutene component of the isomerizer stream. Unreacted n-butenes from the etherification zone effluent can be recycled to the isomerizer where they are converted to isobutene at high selectivity. If the recycle stream contains not only unconverted linear olefins, e.g., n-butenes, but also by-products such as other olefins (e.g., propylene) or paraffins, they have to be removed from the recycle stream, such as by distillation or by taking a slip stream. These removal steps are expensive and can lead to considerable loss of not only the by-products but butenes as well. These losses are larger when the by-products formed are present in higher concentration. Thus, even small improvements in the isobutene selectivity during n-butene isomerization have a major effect on the commercial viability of the process.
Further enhancement of total yield of iso-olefin can be effected by enhancing overall conversion of the n-olefin-containing feedstream. With this object in mind, it would be advantageous to provide a skeletal isomerization catalyst capable of maintaining a high level of conversion as well as high iso-olefin selectivity, even at relatively low temperatures, e.g., no greater than 450.degree. C. and high n-olefin space velocities, e.g., no less than 5, e.g., no less than 70. Such catalyst materials include constrained intermediate pore size zeolites.
These zeolites, exemplified by ZSM-22, ZSM-23, and ZSM-35, are members of a unique class of zeolites. They have channels described by 10-membered rings of T (.dbd.Si or Al) or oxygen atoms, i.e., they are intermediate pore zeolites, distinct from small pore 8-ring or large pore 12-ring zeolites. They differ, however, from other intermediate pore 10-ring zeolites, such as ZSM-5, ZSM-11, ZSM-57 or stilbite, in having a smaller 10-ring channel. If the crystal structure (and hence pore system) is known, a convenient measure of the channel cross-section is given by the product of the dimensions (in angstrom units) of the two major axes of the pores. These dimensions are listed in the "Atlas of Zeolite Structure Types" by W. M. Meier and D. H. Olson, Butterworths, publisher, Second Edition, 1987. The values of this product, termed the Pore Size Index, are listed in Table A.
TABLE A ______________________________________ Pore Size Index Largest Axes of Largest Pore Size Type Ring Size Zeolite Channel, A Index ______________________________________ 1 8 Chabazite 3.8 .times. 3.8 14.4 Erionite 3.6 .times. 5.1 18.4 Linde A 4.1 .times. 4.1 16.8 2 10 ZSM-22 4.4 .times. 5.5 24.2 ZSM-23 4.5 .times. 5.2 23.4 ZSM-35 4.2 .times. 5.4 22.7 ALPO-11 3.9 .times. 6.3 24.6 3 10 ZSM-5 5.3 .times. 5.6 29.1 ZSM-11 5.3 .times. 5.4 28.6 Stilbite 4.9 .times. 6.1 29.9 ZSM-57 (10) 5.1 .times. 5.8 29.6 4 12 ZSM-12 5.5 .times. 5.9 32.4 Mordenite 6.5 .times. 7.0 45.5 Beta (C-56) 6.2 .times. 7.7 47.7 Linde-L 7.1 .times. 7.1 50.4 Mazzite (ZSM-4) 7.4 .times. 7.4 54.8 ALPO.sub.4 -5 7.3 .times. 7.3 53.3 ______________________________________
It can be seen that small pore, eight-ring zeolites have a Pore Size Index below about 17, the intermediate pore, 10-ring zeolites of about 22-30, and large pore, 12-ring zeolites above about 32. It is also apparent, that the 10-ring zeolites are grouped in two distinct classes; Type 2 with a Pore Size Index between about 22.7 and 24.6, and more broadly between about 20 and 26, and Type 3 with a Pore Size Index between 28.6 and 29.9, or more broadly, between about 28 and 31.
The zeolites useful for this invention are those of Type 2 with a Pore Size Index of 20-26.
Alternatively, these zeolites can be distinguished from Type 1 and Type 3 zeolites by their sorption characteristics. Equilibrium sorption data are listed in Table B below. While both Type 2 and Type 3 zeolites sorb more than about 40 mg n-hexane per gram zeolite, the Type 2 zeolites sorb less than 40 mg 3-methylpentane under the conditions specified, in contrast to Type 3 zeolites. Small pore, 8-ring zeolites sorb less than 15 mg of 3-methylpentane per gram of zeolite.
The equilibrium sorption are obtained most conveniently in a thermogravimetric balance by passing a stream of inert gas such as helium containing the hydrocarbon with the indicated partial pressure over the dried zeolite sample held at 90.degree. C. for a time sufficient to obtain a constant weight.
This method of characterizing the Type 2 zeolites has the advantage that it can be applied to new zeolites whose crystal structure has not yet been determined. For mixtures of zeolites with amorphous material or for poorly crystallized samples, the numbers apply only to the crystalline portion.
Thus, zeolites useful for the present invention sorb 30 to 55 mg n-hexane and 15 to 40 mg 3-methylpentane per g dry zeolite in the hydrogen form.
TABLE B ______________________________________ Equilibrium Sorption Data of Medium Pore Zeolites Amount sorbed, mg per g zeolite Type Zeolite n-Hexane.sup.a) 3-Methylpentane.sup.b) ______________________________________ 2 ZSM-22 40 20 ZSM-23 45 25 ZSM-35 50 25 3 ZSM-5 103 61 ZSM-12 52 58 ZSM-57 60 70 MCM-22.sup. 89 79 ______________________________________ .sup.a) at 90.degree. C., 83 torr nhexane .sup.b) at 90.degree. C., 90 torr 3methylpentane
ZSM-22 is more particularly described in U.S. Pat. No. 4,556,477, the entire contents of which are incorporated herein by reference. ZSM-22 and its preparation in microcrystalline form using ethylpyridinium as directing agent are described in U.S. Pat. No. 4,481,177 to Valyocsik, the entire contents of which are incorporated herein by reference. For purposes of the present invention, ZSM-22 is considered to include its isotypes, e.g., Theta-1, Gallo-Theta-1, NU-10, ISI-1, and KZ-2.
ZSM-23 is more particularly described in U.S. Pat. No. 4,076,842, the entire contents of which are incorporated herein by reference. For purposes of the present invention, ZSM-22 is considered to include its isotypes, e.g., EU-13, ISI-4, and KZ-1.
ZSM-35 is more particularly described in U.S. Pat. No. 4,016,245, the entire contents of which are incorporated herein by reference. For purposes of the present invention, ZSM-35 is considered to include its isotypes, e.g., ferrierite, FU-9, ISI-6, NU-23, and Sr-D.
The zeolite catalyst used is preferably at least partly in the hydrogen form, e.g., HZSM-22, HZSM-23, or HZSM-35. Other metals or cations thereof, e.g. rare earth cations, may also be present. When the zeolites are prepared in the presence of organic cations, they may be quite inactive possibly because the intracrystalline free space is occupied by the organic cations from the forming solution. The zeolite may be activated by heating in an inert or oxidative atmosphere to remove the organic cations, e.g. by heating at over 500.degree. C. for 1 hour or more. The hydrogen form can then be obtained by base exchange with ammonium salts followed by calcination, e.g. at 500.degree. C. in air.
Many catalytic conversion processes operate with several reactors rather than only one reactor. Multi-reactor systems can provide greater process control than can be maintained in a single reactor vessel. Moreover, it is often more economical to install several small vessels rather than one very large vessel. Additionally, process flexibility may be increased with multiple reactors so that different operating conditions can be used in each reactor, thus resulting in different product yields, variations in catalyst aging or ultimate life, changes in conversion of feed, or combinations of all of the above.
Multiple reactors may be used in two basic flow configurations. Reactors may be manifolded to operate in parallel or series flow. However, reactors can only be practically used in parallel flow arrangement if the feedstock can be economically converted in a single pass through a catalyst bed. When the catalyst in the reactors requires frequent replacement or reactivation, an extra reactor may be installed and throughput can remain constant during catalyst replacement or reactivation.
Skeletal isomerization operation with the constrained intermediate pore size zeolite catalyst is preferably carried out at relatively high temperatures, e.g., 400.degree. C., and low olefin partial pressures, e.g., 1 atm. Though selectivities for isobutylene are generally high, e.g., greater than 80%, the process can experience relatively low selectivities, e.g., 60%, for the first day or so of operation with fresh or freshly regenerated catalyst on stream.
Accordingly, it would be advantageous to devise an olefin skeletal isomerization process operation whereby the cycle length (time between successive regenerations of catalyst) and catalyst life are increased, while maximizing iso-olefin selectivity during initial operation with fresh or freshly regenerated catalyst.